Their departure comes more than seven years after U.S. combat forces entered, though their departure does not signify the end of all U.S. combat forces in the country.

Another 6,000 U.S. troops must leave Iraq to meet Obama's deadline for the end of U.S. combat operations in the country and the beginning of Operation New Dawn, in which the remaining U.S. forces are expected to switch to an advise-and-assist role.

Video: Milestone not the end

Video: Last U.S. combat convoy leaves Iraq

Video: U.S. combat team: Goodbye Iraq

Video: What's next for the U.S.?

We're keeping the promise that we've made when I began my campaign for the presidency--U.S. President Barack Obama

A public information officer at Joint Base Lewis-McChord, Washington, said it will take a few weeks for all of the 4-2's members to return home. "It is one flight at a time," she said. "We are expecting most of them to be home by mid-September."

As they prepared to depart, some soldiers laughed and some expressed relief at having survived multiple deployments. A few reminisced about having endured firefights and helping carry the bodies of buddies off the field of battle. Many said they would never forget the war.

"The first time you get shot at, it's just, I mean, it wakes you up," said Sgt. Terry Wetzel, the company's senior sniper. "You think, before you come here, that you're an adult, that you're a grown man. But this place will change you."

Wetzel said he was ready to go home. "I feel like we have done as much as we can do here now. It's pretty much up to the Iraqi army and Iraqi police and their government," he said. "We have helped them out as much as we can."

"We put our blood, sweat and tears since we've been here for 12 months and we know we did our job and we know it's not going to be in vain, but there's a lot of excitement right now," said Spc. Don Lanpher as he prepared to depart.

"We're keeping the promise that we've made when I began my campaign for the presidency," Obama said Wednesday in Columbus, Ohio, where he was attending a Democratic fundraiser. "By the end of this month ... our combat mission will be over in Iraq."

Obama said that more than 90,000 U.S. troops have left Iraq in the past 18 months.

"And, consistent with our agreement with the Iraqi government, all of our troops will be out of Iraq by the end of next year," he said.

Former U.S. Ambassador Ryan Crocker told CNN that the United States has plenty of work left to do in Iraq.

"Iraq is still at the beginning of the story of its evolution since 2003," he said, referring to the date of the U.S.-led invasion of the country. "As tired as many Americans may be, this process is still just at its beginning."

In fact, Iraq remains without a functioning government, electricity and other utilities are available only sporadically in the capital, and violence appears to be increasing. At least 48 people were killed Tuesday outside a military recruiting center in Baghdad.

"We're going to have to leave a large footprint behind, and this is not going away for us as an issue," he said.

The State Department is preparing to leave much of that footprint. It will handle many of the responsibilities currently shouldered by the military, increasing its security contractors from 2,700 to nearly 7,000, sources said.

They are expected to work with diplomats and police trainers in some facilities.

The State Department has asked for an additional $400 million to cover the costs, though it was not clear they would get it.

The State Department has asked the U.S. military to leave behind surveillance systems, about 50 bomb-resistant vehicles and a few dozen UH-60 helicopters, a military official said. Though they got a lot of what they wanted, the Pentagon said it could not give them all the helicopters because they are needed in Afghanistan, the official said.